Methods of and apparatus for operating electrical amplifiers



E. H. LOFTIN 1,763,401

METHOD OF AND APPARATUS FOR OPERATING ELECTRICAL AMPLIFIERS Jung 10, 1930.

Filed Nov. 12 1926 fig 5.

. jsvilhziiml' +BLOW of alternating currents.

Patented June 10, 1930 UNITED STATES PATENT OFFICE EDWARD H. LOFTIN, OF NEW YORK, N. Y., ASSIGNOR, BY MESNE ASSIGNMENTS, TO

RADIO CORPORATION OF AMERICA, OF NEW YORK, N. Y., A CORPORATION OF DELA- WARE METHODS OF AND APPARATUS FOR OPERATING ELECTRICAL AMPLIFIERS Application filed November 12, 1926. Serial No. 147,953.

My invention relates generally to the operation of electrical amplifiers, and has for one object ameans for operating amplifiers with circuits resonant to high frequency elec- 5 trical currents, particularlythree electrode vacuum tube amplifiers.

A particular object is the resonant abstraction of high frequency electrical current energy from the output circuit of an amplifier 0 without producing undesirable reactions on a resonant input circuit.

A further object is to resonantly abstract a large percentage of the available energy from the output circuit of an amplifier with- 5 out causing self-generation of oscillations in the amplifier system.

Another object is to maintain uniform results with variation of frequency in using the system to cover a wide range of frequencies, 0 or to cause the results to vary in any predetermined way with frequency.

It is well known that if the input and output circuits of an electrical amplifier are linked so that energy in the outputcircuit is plification of the system can be varied over a wide range of performance dependin upon the amplifying ability of the ampli er per se, the degree of linkage or coupling between 0 the circuits, and the phase relations of the energy returnin to the input circuit to that originally in theinput circuit, and that such systems may even'reamplify or regenerate to such extent that they become self-generators These effects are most pronounced in the well-known'and highly efiicient three-electrode vacuum-tube amplifier, so much so that great difliculty has been encountered in the use of such tubes as amplifiers of alternating currents of sufficiently high frequency for the inter-electrodal or inherent tube capacity toform an effective coupling between the grid and plate circuits of such tubes. It is now well understood that the nature and degree of the reaction of the plate circuit determine the phase and amount of energy escaping to the grid electrode from the amplified energy in the plate circuit through the inherent path in the tube, the return being more than 90 out of phase returned to the input circuit the overall amfor a capacitively reacting plate circuit and thereby causing deamplification, substantially 90out of phase for a non-reactive plate circuit and thus having no influence on amplification, and in phase for an inductively reacting plate circuit and thus causing reamplification, often to the point of self-generation of oscillations. The abstraction of energy from the plate circuit for any purpose is accompanied by a reaction on the plate circuit, and it often happens that the object and mode of abstraction donot result in a reaction that is favorable to results sought, and this has heretofore been particularly true in connection with attempts to resonantly abstract high frequency current energy from the plate circuit, and has resulted in failure to obtain the desired result, all of which is explained in detail in connection with the drawings and the herein described means, the subject of my invention, for overcomingthe diificulty.

Figs. 1, 2, 3, 4, 5 and 7 are explanatory threeelectrode vacuum tube amplifier circuit diagrams and graphs, and Fig. 6 is a circuit diagram of my amplifier system for obtaining the resultsas will be described.

Fig. 1 conventionally shows the three electrode vacuum tube having a tunable circuit 1 forming a portion of the input or grid circuit and a tunable circuit 2 forming a portion of the output or plate circuit, it being well understood that on account of the high impedance of the plate and grid circuit paths through the tube that these tunable'circuits both act fairly independent of the rest of the system in the matter of resonance effects.

There is conventionally shown a source of alof frequency i that the amplified energy forced by this reactance through the inherent ,rent intensity and abscissae f represent frequency of bothinput current and circuit 2 adjustment. The broken'line marked os-- cillation line represents that point of reamplification beyond which the system becomes a self-generator of oscillations, the dotted portion of curve a above this line indicating that throughout the region covered the system remains in a state of oscillation, the existence or non-existence of this oscillation region depending, however, on the frequency being high enough for the tube capacity to be an effective coupling, the amplifying ability of the tube, and the choice of constants in making up the tunable circuits,'but being only too readily attained with modern tubes at the frequencies used in radio broadcast practice.

It is seen from curve a that with circuit 2 adjusted to the incoming frequency f the plate current energy falls away most substantially and more so than if circuit 2 were adjusted by the same amount above the arbitrary critical frequency f the reason for this difference bein that at the lower and resonant frequency the plate circuit is nonreactive, producing a return of energy to the grid substantially 90 out of phase with the grid energy, and therefore neither aiding nor opposing in the matter of reamplification, while at the frequency above f the reaction is still inductive, and the fed-back energy aids the grid energy to produce reamplification.

It is also enerally agreed in the art that a vacuum tu e in a highly critical reamplification state produces amplification in the late circuit 0 hundreds or more times thatmtroduced from the original source into the grid circuit compared to an amplification of about eight for thesame tube in the non-reamplifying state. It is therefore obvious that if energy could be resonantly abstracted from the plate circuit and utilizedwhile the system is in the highly critical state that enormous amplification would be possible in cascaded vacuum tube systems compared to that now had; as for instance in two stages the ratio Ihight be of the order of ten thousand or more to the twenty-five'to thirt now had. But this has never been accomplis ed for the reason that any attempt to resonantly abstract the energy intro uces the reaction of the abstracting circuit onto the plate circuit to alter the critical plate circuit reaction responsible for the high reamplification, for it is obvious that coupling the tunable circuit 2 to the plate circuit, as shown in Fig. 2 for the purpose of linking a second tube to the first, does not alter the reaction conditions on the plate circuit in character, but only in intensity in keeping with the percentage coupling employed in the linking, so that the plate circuit current intensit can be represented by a second curve b in ig. 3 following 'to lessen the tendency of broadcast radio receivers to entering the undesirable oscillatingr state. v

he three electrode vacuum tube is sometimes referred to as a uni-directional device in the sense-that it is said to pass energy forward from a preceding tuned circuit to a succeeding tuned circuit and act in its intermediate position to prevent the reaction of the second circuit back onto the first. Without the provision of some means or effect havin the function of stoppingup the inherent bode in the tube, so to speak,-or tube capacity, it is obvious that such alleged function is amplifiedly erroneous to the extent that the tube is an amplifier, for not only is there a reaction coupling in the inherent tube capacity between the two circuits just as between any two coupled electrical circuits, but with the tube as the linking element the reaction is magnified to the extent of not only amplification, but the reamplification above pointed out; in fact the conditions are so bad that it is impossible to link two circuits through a vacuum tube uncorrected for the tube capacity and have them act in. any wise as cascaded resonant circuits in a step-by-step system.

Examining this situation a little more.

clearly, it is seen from Fig. 3 that curve I) shows the plate circuit ener is a maximum when the coupled circuit 0 Fig. 2 is at'the .off frequency f and therefore there is no resonant transfer of energy at this point. If there was such resonant transfer a large share of the enormous energy of reampllfication would get over to the second tube and radio receivers would be many fold more efiicient than we know them in practice. Still observing curve b we see that when coupled circuit 2 is adjusted to the frequency of the incoming energy, as well as to tuned grid circuit 1, at which adjustment there can be a resonant transfer of energy, the available plate circuit energy has fallen to a ver low. value compared to that at the .pea c, and this fact,

been made loose to prevent curve b from ri'sing above the oscillation line, results in nates over or swamps the much lesser effect at the resonant point. In other words, the varying of the second circuit in such a system does not result in tuning in the sense that it is understood in the art and often erroneously so termed in such systems, but in producing a varying reaction on the plate circuit that chan es the amplifying ability of the vacuum tube; and curves a and b of Fig. 3 are not resonance curves as understood in the art, but curves of varying amplifying ability of a vacuum tube in no way related'to resonance effects but rather phase effects.

It is well appreciated that the reamplifying type of vacuum tube system, such as shown .in- U. S. Patent No. 1,113,149 to Armstrong, is highly selective, but it is obvious that thisis due to the combination of the selective resonant input to the grid circuit, and the frequency discriminative reaction production of the a justable circuit in the plate circuit, and this same effect persists in cascaded coupled adjustable circuit systems, such as shown in U. S. Patent 1,087 ,892 to Schloemilch, et al., Fig. 3, and U. S. Patent 1,17 3,079 to Alexanderson, it now being obvious that Alexandersons idea that he was improving selectivity through step-by-step resonance instead of the reaction effect of the Armstrong patent was erroneous in view of his having done nothing to close up the troublesome tube capacity path.

So far I have considered the action that takes place at but one frequency. If the frequency of input to circuit 1 of Fig. 2 be varied over a wide range of frequencies, and circuit 1 tuned in consona'njce, as is the case in broadcast and like radio receivers, then the inductive coupling between the plate circuit and circuit 2 will not remain constant as circuit 2 is adjusted to create the loudest signal by critical reaction production, but will increase with frequency as shown graphically by the curve X in Fig. 4, where ordinates represent energy E transferred, or the accompanying reaction X produced by the variable circuit 2 on the plate circuit, and abscissae represent frequency f. With such an upward sloping curve it is seen that if the inductive coupling is made sufficiently loose to produce at the higher frequencies (shorter wave lengths) of a receiver a reaction curve just below the oscillation line like curve b of Fig. 3, then at the lower frequencies (longer wave lengths) the reaction curve will fall well below the oscillation line in accordance with curve X in Fig. 4, and such a receiver becomes extremely ineflicient at the longer wave lengths. If the coupling is tightened to improve the conditions on the longer wave lengths then the reaction curve of Fig. 3 rises above the oscil- I lation line to produce oscillations with the resulting squeals so distasteful in broadcast receivers.

Such are the undesirable conditions that are obtained in the system of the Alexanderson patent mentioned, and in somewhat different degree in the Schloemilch, et a1., patent also-mentioned, the difference being that in Fig. 3 of Schloemilch there is included in the plate circuit of the radio frequency amplifying tube a a choke coil 1" which, though put in for another purpose, produces a capacitive reaction in the plate circuit by reason of its distributed though indefinite capacity, and which capacitive reaction therefore opposes any inductive reaction on the plate circuit of the adjustable circuit containing the variable condenser 91.. The effect of thisi's illusable circuit on the plate circuit, and the curve X represents the decreasing with frequency capacitive reaction of the choke coil r, the curve being shown below the line to illustrate the opposed phase of the capacitive, reaction with respect to the inductive reaction. The dotted curve X represents the resulting overall reaction with frequency in the plate circuit due to the combination of the two reactions. While I have shown the curve X as passing through the zero line, or changing from capacitive to inductive, of course there is no way of knowing in the absence of definite specifications asto values in the Schloemilch patent 'ust what position such a curve would take or any particular band of frequencies.

My invention is directed particularly at overcoming the above recited difficultics and providing a system that will permit of actual step-by-step tuning and resonant transfer of energy, and maintaining this condition throughout a wide band of frequencies, such as the 500 kilocycles to 1500 kilocyclcs of broadcasting, or deviating under control from these conditions to any desired pro-determined degree, and the manner in which I accomplish the results is illustrated in and explained with the aid of Fig. 7;

For the purpose of description I have chosen a portion of a conventional radio receiver in which I include the features necessary to practice my invention, the vacuum tubes VT, and VT Fig. 6, representing two radio frequency amplifiers followed by a (letecting vacuum tube VT the filament and plate circuits being energized in a well-known manner from the several sources marked Fig. (i,

15 S of condenser C as through a cam 0, so that +A-B, -A, +13 low, and +B high. There is shown an antenna A connected to a tunable circuit 1 in the grid circuit of tube VT The plate circuit of tube VT is shown to include a condenser C in series with the primary winding L of a high frequency transformer, and on account of the condenser (l -interrupting the plate circuit for direct current flow it is shunted by radio frequency choke coil L preferably. of low distributed capacity with respect to condenser C An adjustable circuit containing secondary winding L and variable condenser C is coupled to the plate circuit of VT to collect amplified energy for tube VT I associate coil L with the shaft rotation of the shaft to vary condenser C will cause any desired movement of coil L about pivot P to produce any predetermined change in coupling with frequency between theplate circuit and the adjustable circuit as it is tuned in consonance'with the frequency to be received and circuit 1. Of course, I might vary the coupling mechanically in a number of ways other than that specifically illustrated, and do not intend any limitations in this respect by reason of the specific illustration. In my co-pendi'ng application,

Serial Number 77,445, filed December 24,

1925, I described a method of producing a like result electrically through the combination of electromagnetic and electrostatic coupling between the circuits, so that this invention is confined to avoiding the use of such combined coupling and producing the result mechanically.

It is now obvious that with the means described I can avoid the usual result had with ordinary fixed inductive coupling illustrated by the upward sloping curve X in Fig. 4, and can make this curve slope either up (curve X Fig. 7), or down (dotted curve X' Fig. 7 or remain horizontal with frequency by proper design of a cam on the shaft of condenser C or any other simple mechanical expedient cooperating with the condenser shaft or other varying mechanical element, and the use I may make of such an arrangement is illustrated in Fig. 7. l

For instance, suppose the-condenser C is selected of such value as to create a capacitive reaction with wave length in the plate circuit represented by the curve X and I select such coupling between the circuits and vary it mechanically so that the inductive reaction of the tunable circuit is like the curve X that is, equal to and opposed to curve X then the resulting reaction with frequency of the plate circuit will be zero, or it will be non-reactive, as represented by the line X so that noreamplifica-tion takes place. As a consequence the tunable circuit can be actually tuned in consonance with the received frequenc and circuit 1 to obtain a resonant trans er of energy to tube VT and the circuit no longer acts as a mere reaction producer to change the amplifying ability of tube VT and prevent use of resonance. It is also obvious that I can make the coupling as tight as desired and select condenser C accordingly and get a resonant transfer of very high efiiciency, so that even though the energy in the plate circuit is much less due to pure amplification of the tube only, and no rearnplification, the efficiency may be higher than with the reamplifying type of system, and there is nc trouble from undesirable oscillations due to complete removal from that critical ad ustment that is necessary with reamplification.

If condenser C be properly chosen to produce a reactance curve X in Fig. 7 less than reactance curve XL, then reamplification to any desired degree, as represented by dotted curve X, can be left in the system, or the arrangement can be reversed to obtain deamplification if desired, and by properly varying the coupling with frequency these results can be made constant or to vary under control in any desired manner.

In connection with the plate circuit of tube VT I have shown the same arrangement for delivery of energy to detector tube VT except I have shown a choke coil L having distributed capacity shown in dotted lines in lieu of the condenser C If a choke coil be designed to have the necessary distributed capacity to produce the required capacitive reaction for any desired result it is obvious that such arrangement may be employed and avoid the use of condenser 0 and choke coil L Having fully described my invention, and not wishingv to be limited therein by the specific apparatus. employed for illustration purposes, I claimt 1. The combination of an amplifier having an output circuit, a tunable circuit coupled to said output circuit, a 'capacitively reacting element in said output circuit in series with said coupling, and mechanical means for varying the coupling between said circuits as i said tunable circuit is tuned in such a' way that the inductive reaction of said tunable circuit on said output circuit maintains with frequency a substantially constant relation to the capacitive reaction of said, capacitivcly reacting element. l

2. In an amplifier system including a threeelect-rode vacuum tube having a tunable circuit coupled to its output circuit the method of controlling the regenerative reaction through the internal capacity of said tube caused by the inductive reaction on said output circuit created by said tunable circuit as it is tuned, which comprises creating a capacitive reaction in said output circuit, and adjusting and mechanically control-ling the coupling between said circuits as said lIiH tunable circuit is tuned to produce an inductive reaction on said outputcircuit bearing a predetern'iincd relation with frequency to said capacitive reaction.

3. In an amplifier system including a threeelectrode vacuum tube having a tunable circuit coupled to its output circuit the method of preventing regenerative reaction through the internal capacity of said tube by the inductive reaction on said output circuit created by said tunable circuit as it is tuned, which comprises creating a capacitive reaction in said output circuit, and adjusting and mechanically controlling the coupling between said circuits as said tunable circuit is tuned to produce an inductive reaction on said output circuit substantially equaling and r pposing said capacitive reaction with frequency. V

4. The combination of an amplifier having an output circuit, a tunable circuit coupled to ditions of said tube the adjustment 0 said output circuit, a capacitively reacting element in said output circuit in series with said coupling, and mechanical means for varying the coupling between said circuits as said tunable circuit is tuned in such a wa that the inductive reaction of said tunable circuit on said output circuit maintains with frequency a substantially equal and opposing relation to the capacitive reaction of said capacitively reacting element.

5. The combination of a three-electrode vacuum tube amplifier having an output circuit, a circuit tunable over a given wide range of frequencies so closely coupled to said output circuit that under normal operating-consaid tunable circuit will cause the system to selfgenerate oscillations, a capacitlvely reacting element in said output circuit in series with said coupling of such value as to chan e the reaction of said plate circuit to any egree below which oscillations will not occur while maintaining said close coupling, and mechanical means for varying said coupling as said tunable circuit is tuned so as to maintain said non oscillating condition over said wide range of frequencies.

6. In a system for efliciently selectively amplif ying high frequency electrical currents over a wide range of ,frequencies including a three electrode vacuum tube amplifier having inherent capacity between its electrodes, a variable period circuit connected to the input electrodes of said tube, an output circuit, a second vacuum tube, a second variable period circuit of like frequency'range as said first variable period circuit in the input circuit'of said second tube so closely coupled to said output circuit that its reactions on said output circuit as it is varied to follow the varying of the input circuit of said first tube inevitably include that reaction which unmodiliedly acting through said inherent tube capacity is of such phase and potential as to produce oscillation, means for annulling a sufficient amount of the effect of said reaction while maintaining said close coupling that oscillation is prevented, and mechanical means associated with a variable element of one of said variable circuits for varying said coupling with frequency to control sald oscillation producing reaction to follow any variation with frequency of said annulling effect, whereby said oscillation prevention is effective to substantially the same degree throughout said wide range of frequencies.

7 The combination with an amplifier system including a three-electrode vacuum tube having a mechanically operated tunable circuit coupled to its output circuit of means for controlling the regenerative reaction through the internal capacity of said tube caused by the inductive reaction on said output created by said tunable circuit as it is tuned, including a capacitively reacting element in said output circuit, and mechanical means'linked with said mechanically operated tuning means adapted to adjust and controlrthe coupling between said circuits as said tunable y circuit is tuned to produce an inductive reaction on said output circuit bearing a predetermined relation with frequency to the capacitive reaction with frequency of said capacitively reacting element.

EDWARD H. LOFTIN. 

